It is known in the art to introduce small quantities of ammonia, NH.sub.3, into a combusted gas stream containing nitrogen oxides such as nitric oxide, NO, and nitrogen dioxide, NO.sub.2, in order to form molecular nitrogen gas, N.sub.2, and water vapor, H.sub.2 O, thereby reducing the concentration of noxious nitrogen oxides.
In U.S. Pat. No. 3,900,554 issued to Lyon on Aug. 19, 1975, a method of reducing nitric oxide is disclosed wherein ammonia is introduced into a combusted gas stream in a specified mole ratio to the amount of nitric oxide contained in the combusted gas stream. In order for the reaction to occur, this method specifies that the combusted gas stream must be at an elevated temperature of between 704.degree. to 1073.degree. C. However, Lyon does not teach or suggest that his method is applicable to a diesel engine since a diesel engine's combusted gas stream temperature typically ranges from 100.degree. to 600.degree. C., depending on the load, thus never attaining the higher temperature range required by Lyon in order for the ammonia to react with the nitric oxide. Further, the amount of ammonia delivered to the combusted gas stream may be continually either inadequate or excessive if it is governed by analysis of the combusted gas stream. Combusted gas analysis by a conventional nitrogen oxide sensor involves a significant time lag before any change in the amount of nitrogen oxides produced is detected and the corresponding amount of ammonia delivered can be adjusted whereas the amount of nitrogen oxides in the combusted gas stream varies according to the engine load which can more rapidly change.
Another approach to the reduction of nitrogen oxides is disclosed in U.S. Pat. No. 3,846,981 issued to Paczkowski on Nov. 12, 1974 in which the combusted gas stream is heated by an afterburner which is maintained within a temperature range of 704.degree. to 1038.degree. C. The heated combusted gas stream is thereafter cooled by ambient air and then contacted with a gaseous nitrogen-containing compound. The amount of nitrogen-containing compound contacting the combusted gas stream is regulated in response to exhaust gas temperature or velocity, operating motor vacuum, or in response to a nitrogen oxide sensor in the combusted gas stream. However, the Paczkowski patent does not teach or suggest what quantity of nitrogen-containing compound should contact the combusted gas stream.
In order to enable ammonia to react with nitrogen oxides at typical combusted gas stream temperatures of a diesel or spark-ignited engine, it is also known to mix gaseous ammonia into the combusted gas stream, in proportion to the amount of nitric oxide contained in the combusted gas stream, and then route the gaseous mixture to a catalytic reactor. Such an approach is disclosed in U.S. Pat. No. 4,188,364 issued to Gladden on Feb. 12, 1980. The use of a catalytic reactor enables the nitrogen oxides and ammonia to be adsorbed on the catalyst bed where the reaction can occur at the normal combusted gas stream temperatures rather than at the elevated temperature range required by the Lyon or Paczkowski patents. As mentioned before, however, a conventional nitrogen oxide sensor has the disadvantage of relatively slow response as well as high cost and even unreliability if the sensor is not frequently serviced. Further, the reactor temperature varies according to engine load such that the reactor may at times be either too cold or too hot for the ammonia to react on the catalyst bed and reduce nitrogen oxides. Thus the supply of ammonia to the reactor may at times be wasted.
In summary, the above patents do not teach or suggest a way of reliably and instantaneously metering only the required amount of ammonia to the combusted gas stream in order to efficiently reduce the amounts of noxious nitrogen oxides while also conserving the supply of ammonia.
The present invention is directed to overcoming one or more of the problems as set forth above.